It is known to those skilled in the art that the amination of 4,5-dichloro-2-phenyl-3(2H)-pyridazinone leads through the two simultaneous reactions: ##STR1## to the formation of commercial PYRAZON, that is a product constituted by a mixture of two isomers, one of which, 5-AMINO-4-CHLORO-2-PHENYL-3(2H)-PYRIDAZINONE (also referred to hereinafter by the abbreviation PCA) is active as a selective weed-killer for agricultural use while the second, 4-AMINO-5-CHLORO-2-PHENYL-3(2H)-PYRIDAZINONE (also referred to hereinafter by the abbreviation ISO-PCA) does not have any herbicidal activity (and the presence of which is therefore superfluous, if not harmful, when it is applied to the soil together with the first).